1. Technical Field of the Invention
The present invention relates generally to digital color image sensors, and specifically to a two-color photo-detector and related circuitry for use in digital color image sensors.
2. Description of Related Art
Digital color image sensors are predominately of two types: CCDs (Charge Coupled Devices) and CMOSxe2x80x94APS (Complimentary Metal Oxide Semiconductorxe2x80x94Active Photo-detector Sensors). Both types of sensors typically contain an array of photo-detectors (e.g., pixels), arranged in rows and columns or arranged in another pattern, that sample color within an image. Each photo-detector measures the intensity of light within one or more ranges of wavelengths, corresponding to one or more perceived colors. In addition, both types of sensors may include a color filter array (CFA), such as the CFA described in U.S. Pat. No. 3,971,065 to Bayer (hereinafter referred to as Bayer), which is hereby incorporated by reference. With the Bayer CFA, each photo-detector sees only one wavelength range, corresponding to the color red, green or blue.
A sensor fitted with a Bayer CFA produces a mosaiced image that is sampled in both the color space and in the spatial domain. The sampling process produces aliasing artifacts in both the color space and in the spatial domain. For example, since only portions of the full color spectrum are sampled at any given photo-detector location (depending on the CFA), it is impossible to accurately reconstruct the true color of an image, thereby producing color space aliasing artifacts. In addition, since high spatial frequencies in the original image are sampled at too low of a frequency, the original high frequencies in the image cannot be restored later on through image processing, thereby producing spatial domain aliasing artifacts. One solution to the color space and spatial domain aliasing artifact problems is alternative sensor designs.
In one alternative sensor design that does not use a CFA, a special prism separates and captures the three primary colors at the same photo-detector location, as is described by Richard F. Lyon in xe2x80x9cPrism-Based Color Separation for Professional Digital Photography,xe2x80x9d Proceedings of 2000 PICS Conference, ISandT, p. 50-54, which is hereby incorporated, by reference. However, the cost of the prism and optics is extremely high. In addition, the need to manually align, in both X and Y, the three sensors and optics to less than a fraction of the width of a photo-detector, which is on the order of 3 microns, is prohibitive for many imaging applications.
Another type of sensor design is described in both U.S. Pat. No. 5,998,806 to Stiebig et al. and U.S. Pat. No. 5,965,875 to Merrill, which are both hereby incorporated by reference. The Stiebig et al. and Merrill sensors stack three separate color photodiodes, and electrically connect the photodiodes together to form one photo-detector capable of sensing all three primary colors at a single spatial location. However, both the Stiebig et al. sensor and Merrill sensor include common anodes, such that any current coming out of a three-color photo-detector location is a combination of more than one photodiode current. Therefore, in order to measure the differences in current coming out of each of the photodiodes, a significant amount of extra circuitry is required, which can be both cost prohibitive and area prohibitive.
A further alternative sensor design is described in an article by K. M. Findlater et al. entitled xe2x80x9cBuried Double Junction Photo-detector Using Green and Magenta Filters,xe2x80x9d 1999 IEEE Workshop on CCDs and Advanced Image Sensors, p. 60-64, which is hereby incorporated by reference. Instead of a xe2x80x9cthree color photo-detector,xe2x80x9d as described in Stiebig et al. and Merrill, the Findlater article describes a xe2x80x9ctwo color photo-detector.xe2x80x9d In the Findlater sensor, each photo-detector includes two back to back photodiodes resident in the bulk silicon. In addition, a non-Bayer color filter array (CFA) mosaic covers the Findlater sensor. Thus, for every two photo-detectors, four different color values are extracted. Although the Findlater design provides more accurate color reconstruction as compared to a sensor fitted with a xe2x80x9cBayerxe2x80x9d CFA, the color separation of the two bulk photodiodes is poor, since the colors are differentiated only by the differences in absorption with wavelength. In addition, with the Findlater design, the photo-detector itself is quite large due to the fact that two bulk photodiodes and six MOSFETs are integrated into the area of each photo-detector, adding both area and cost.
A further alternative sensor design is described in commonly assigned U.S. Pat. application Ser. No. 10/086,125, filed concurrently herewith, which is hereby incorporated by reference. The sensor design uses elevated two-color photo-detectors with a non-Bayer CFA mosaic. Each photo-detector location contains two sub-photo-detectors, each sensing a different color. One of the sub-photo-detectors of each photo-detector location is elevated above a dielectric layer to electrically isolate the sub-photo-detectors from each other. However, as with the Findlater design, the size of each photo-detector is large due to the number of transistors required to drive each photo-detector. In addition, for a similar area, higher spatial resolutions may be produced using a standard, single-color photo-detector design. Therefore, what is needed is a digital image sensor capable of sensing more than one color at a single photo-detector location with reduced photo-detector area.